Drain-to-Source punch-through breakdown in MOS transistors occurs due to electric field penetration between the drain and source regions. Punchthrough breakdown generally occurs in the bulk region of the substrate at a level slightly below the depth of the source and drain junctions. Since punch-through breakdown has become one of the primary limiting performance factors in submicron MOS transistors, numerous techniques have been developed for suppressing this phenomenon. Deep, high-energy boron implants have been the principal method for controlling the punch-through effect in submicron N-channel MOS transistors. Normally, the deep boron implant is made in the P-well region using a photoresist mask to shield N-well regions. For N-channel devices the higher the concentration of boron in the bulk region, the higher the punch-through breakdown voltage. For P-channel devices, punch-through breakdown generally occurs near the channel surface due to the counter-doping caused by the use of a boron threshold voltage adjustment implant. For CMOS memory applications, P-channel punch-through breakdown is normally not a serious problem because some ninety percent of the transistors in the circuitry are N-channel devices. Consequently, the short-channel effect can be minimized in P-channel devices merely by increasing channel length, without significantly increasing die size.
AT&T Bell Laboratories has developed a process for performing a blanket boron punch-through implant, which eliminates the need for a photoresist mask. Basically, the process involves implanting boron in the presence of a silicon dioxide layer created in a high-temperature steam ambient that is used to protect the newly-created N-well from the subsequent P-well boron implant. Following a high-temperature well-drive step, an additional blanket boron implant is performed for punch-through prevention purposes. Although this process is noteworthy, it does have several disadvantages. Firstly, the silicon dioxide layer must be extremely thick in order to effectively prevent the high-energy boron implant from counter-doping the N-well region. Secondly, since the punch-through implant is performed prior to field oxide growth, the punch-through boron implant will not serve as an effective field isolation implant due to the segregation property of boron. By the segregation property of boron is meant the tendency for boron to migrate to oxide during an oxidation step. Thirdly, the high temperatures required for field oxide growth will cause the narrow profile of the implanted boron impurity to dramatically widen, decreasing its effectiveness as a punch-through implant.
What is needed is a punch-through implant process that combines the benefits of unmasked implants with the feature of accurate implant positioning. In addition, it would be ideal if the punch-through implant could double as a field isolation implant,